The present invention relates to a process for preparing a flexible printed-circuit board, and more particularly to a process for preparing a flexible printed-circuit board covered patternwise with a polymer film.
In recent years, the demand for the flexible printed-circuit board is increasing in a wide range of electric and electronic machines for industrial and household uses. The reason is that because of an excellent flexibility and light weight, the flexible printed-circuit board provides many advantages such as decrease of weight and volume and increase of freeness of spacial wiring, in electric and electronic machines which are subject to spacial restriction in designing electric wiring.
The flexible printed-circuit board is prepared by etching or plating treatment in the same manner as the preparation of usual rigid printed-circuit board. However, the flexible printed-circuit board has the advantage that in its preparation these treatments can be continuously conducted, differing from the rigid printed-circuit board. That is to say, the preparation of the flexible printed-circuit board can be made by a roll to roll operation, but not a piece by piece operation as in the case of the rigid printed-circuit board where the board is treated in the form of discrete sheet, since the flexibility is maintained throughout the preparation from a copper clad film which is the starting material to the end product. The possibility of the continuous preparation largely contributes to the reduction of the cost of printed-circuit board. The second peculiar point to the flexible printed-circuit board which is different from the rigid printed-circuit board is that after forming a conductor pattern in a conductor layer by etching or plating treatment, the surface is patternwise covered with a polymer film as a cover layer, in other words, the whole surface excepting the desired portions containing the portions to which electronic components are connected later in the conductor pattern is covered with a polymer film, in order to improve the flexibility which is the most important characteristic of the flexible printed-circuit board. A polyester film and a polyimide film are usually employed as the polymer film for covering. Breaking of the conductor pattern due to folding is remarkably decreased by providing the cover layer as compared with the case of providing no cover layer. It is considered that the effect of improving the flexibility produced by providing the cover layer is much based on the lowering of the angle or curvature of folding of the conductor pattern itself at the time of folding. Therefore, it is of course desirable that the cover layer is as thick as possible, so far as the merit of being light weight of the flexible printed-circuit board is not contrary to other properties. Although the covering with a polymer film also has the secondary effects of securing the electric insulation of the conductor pattern and of protecting the surface from corrosion as the case of the rigid printed-circuit board, the main purpose of providing the cover layer resides in improving the flexibility.
Hitherto, it has been a common practice to provide the cover layer by laminating a polymer film such as a polyester or polyimide film from which the desired portions as mentioned above is previously removed by means of punching, onto the surface of a flexible printed-circuit board with an adhesive. However, in this laminating process, it is very difficult or troublesome to laminate the film to the surface of the board at an exact position thereof, and in case of a flexible printed-circuit board of high density and high accuracy, the lamination has been impossible. Also, since it is more difficult to continuously conduct the positioning, the lamination has had to be conducted in the form of discrete sheet and the roll to roll process which is the merit on the production cost of the flexible printed-circuit board, has not been adopted.
In recent years, in order to solve such problems, there has been proposed a process in which a polymer in the form of ink is applied imagewise to the surface of the board by a screen process printing and is then set. However, this process is still insufficient for the preparation of the flexible printed-circuit board of high density and high accuracy, and also must be done using a discrete board. Moreover, it is essentially difficult to apply the ink in thick onto the pattern projected of the board, since this process is a printing of an ink having a flowability onto the patterned surface. Therefore, the primary purpose of improving the flexibility, i.e. the folding endurance, cannot be sufficiently attained.
It is an object of the present invention to provide a process for preparing a flexible printed-circuit board.
A further object of the invention is to provide a process for easily preparing a high density, high accuracy flexible printed-circuit board having an excellent flexibility.
These and other objects of the invention will become apparent from the description hereinafter.